1. Field of the Invention
The present invention relates to an antenna structure for use in a mobile communication device exemplified by a telephone terminal of a cellular system or a digital cordless telephone system. The present invention relates particularly to an antenna structure for use in a mobile communication device, which is capable of obtaining good antenna gain without degradation thereof even if a finger, etc., of a user is in contact with an antenna portion of the mobile communication device by means of a structure with which a large distance can be provided between the antenna portion protruding from a casing of the communication device and a surface of a cap covering the antenna portion. The present invention further relates to a mobile communication device having the same antenna structure.
2. Description of Related Art
In general, a mobile communication device exemplified by a telephone terminal of a cellular system or a digital cordless telephone system, etc., includes a portable casing and an antenna provided thereon for transmitting/receiving an electromagnetic wave.
A typical example of a conventional mobile communication device and an antenna structure thereof will be described with reference to FIG. 10a to FIG. 11b. 
FIG. 10a, FIG. 10b and FIG. 10c show an outer appearance of the conventional mobile communication device in which FIG. 10a is a back side view, FIG. 10b is a left side view and FIG. 10c is a front view.
FIG. 11a and FIG. 11b are cross sectional front views of a main portion of the conventional radio communication device, illustrating a general antenna structure thereof, in which FIG. 11a shows a state where the antenna is retracted in a casing and FIG. 11b shows a state where the antenna is extended.
The conventional mobile communication device 1 shown in FIG. 10a to FIG. 10b is for example, a telephone terminal of a cellular system or a digital cordless telephone system, etc., and has a structure including a casing 2 composed of a display side casing portion 3 and a key side casing portion 4 hinged to the display side casing portion 3 to make the casing 2 foldable and housing components of the radio communication device, which are not shown, and an antenna 10 protruded externally from a surface of the casing 2 for transmitting/receiving an electromagnetic wave.
The casing 2 constitutes a body of the mobile communication device 1 and, as shown in FIG. 10a to FIG. 10c, the display side casing portion 3 having a display means 3a such as a liquid crystal display panel and the key side casing portion 4 having input means 4a such as key buttons are hinged each other by a hinge portion 5.
By hinging the display side casing portion 3 to the key side casing portion 4 in this manner, it is possible to fold the mobile communication device 1 when the latter is out of use to thereby make the whole radio communication device compact enough to easily keep it in a bag or a pocket of a jacket.
On the other hand, when the mobile communication device 1 is used, the mobile communication device can be opened by opening the casing portions 3 and 4 from each other, so that it is possible to easily perform a communication and/or a key input operation.
Such foldable casing structure, which can give superior portability and operability to the communication device, has been widely used in the compact communication devices such as telephone terminals of cellular systems or digital cordless telephone systems.
An antenna mounting portion 6 extending into the casing 2 is formed on an upper edge side of the display side casing portion 3 of the casing 2 and an antenna 10 is mounted in the antenna mounting portion 6.
As shown in FIG. 11a and FIG. 11b, the antenna 10 includes a whip antenna 11, a helical antenna 12, a cap 13 and a feeding portion 15.
The whip antenna 11 is a rod antenna provided inside the display side casing portion 3 of the casing 2.
The whip antenna 11 is constituted with a flexible antenna element in the form of a conductive wire and an insulating member covering a surface of the flexible antenna element and can selectively extensible externally of the casing through the antenna mounting portion 6 and the feeding portion 15.
The helical antenna 12 is arranged on a top end of the whip antenna 11.
The helical antenna 12 is insulated from the whip antenna 11 and is always protruded externally of the display side casing portion 3 with an assistance of the antenna mounting portion 6 as a stopper.
The cap 13 is formed of an insulating material and covers the surface of the helical antenna 12.
The insulating material of the cap 13 is usually a synthetic resin such as ABS resin.
The feeding portion 15 is a cylindrical metal member and is screwed inside the antenna mounting portion 6 formed on the upper end of the display side casing portion 3 as shown in FIG. 11a. 
The antenna 10 is fixedly mounted on the casing portion 3 by screwing the feeding portion 15 into the antenna mounting portion 6.
In the state where the antenna is retracted, the feeding portion 15 becomes in contact with a bottom surface of the helical antenna 12 (see FIG. 11a) and, in the state where the antenna is extended, the feeding portion 15 becomes in contact with a lower end portion of the whip antenna 11 (see FIG. 11b), so that the antennas 11 and 12 operate with using these contact points as feeding points thereof, respectively.
More specifically, when the antenna is retracted, the bottom surface of the helical antenna 12 protruding from the display side casing portion 3 is in contact with the feeding portion 15 as shown in FIG. 11a, so that the helical antenna 12 works as an antenna of the mobile communications device 1.
On the other hand, when the antenna is extended, the lower end of the whip antenna 11 is in contact with the feeding portion 15 as shown in FIG. 11b, so that the whip antenna 11 works as the antenna of the mobile communication device 1.
Therefore, when the mobile communication device 1 is a telephone terminal of a cellular system using a frequency band of 800 MHz (or 1.5 GHz), the helical antenna 12 of the terminal in the state where the antenna is retracted and the helical antenna 12 is in contact with the feeding portion 15 operates as an antenna resonating at 800 MHz (or 1.5 GHz) and the whip antenna 11 in the antenna extended state in which the whip antenna is in contact with the feeding portion 15 operates as an antenna resonating at 800 MHz (or 1.5 GHz).
When the mobile communication device 1 is a telephone terminal of a digital cordless telephone system using a frequency band of 1.9 GHz, the helical antenna 12 in the retracted state of the antenna of the terminal operates as an antenna resonating at 1.9 GHz and the whip antenna 11 in the antenna extended state operates as an antenna resonating at 1.9 GHz.
As described, in the conventional antenna structure of a mobile communication device, the extensible whip antenna and the helical antenna can be used in the retracted or extended state of the antenna and it is possible to perform radio communication by means of the helical antenna protruding from the casing even in the retracted state of the antenna in which the whip antenna is not extended.
Such antenna structure of the mobile communication device is disclosed in, for example, JPH8-316724A.
Besides, with recent expeditious development of the data communication technology exemplified by the Internet, the mobile communication device can have not only the conventional function of telephonic communication but also the transmission/receiving function of data such as document data and/or image data and there have been demands of increase of the size of a liquid crystal display portion for displaying the document data and the image data and improvement of the performance of the liquid crystal display portion.
However, since the antenna structure of the conventional mobile communication device is arranged on the display side casing portion as described previously (see FIG. 10a to FIG. 10c), an arrangement of display means such as a liquid crystal display portion provided on the side of the display side casing portion is restricted by the existence of the antenna, so that there is provided a problem that the increase of the size of the display means and the improvement of performance thereof are restricted.
In order to solve the problem of the mobile communication device having the antenna structure on the display side casing portion to thereby make the increase of the size of the display means on the side of the display side casing portion and the improvement of the performance thereof possible, a mobile communication device having an antenna structure on a key side casing portion of a foldable casing has been provided.
FIG. 12a to FIG. 12c show an outer configuration of such mobile communication device having an antenna structure on a key side casing portion of a foldable casing in which FIG. 12a is a back side view thereof, FIG. 12b is a left side view thereof and FIG. 12c is a front view thereof.
As shown in these figures, the mobile communication device 1 has a foldable casing 2 including a display side casing portion 3 and a key side casing portion 4 hinged to the display side casing portion 3 and an antenna 10 provided on the side of the key side casing portion 4.
Incidentally, a construction of the mobile communication device 1 other than the antenna 10 is similar to that of the conventional mobile communication device shown in FIG. 10a to FIG. 11b. 
In the antenna structure in which the antenna 10 is provided on the side of the key side casing portion 4, it is possible to concentrically arrange a substrate in the key side casing portion 4 of the foldable casing and to use the display side casing portion 3 as dedicated to a liquid crystal display portion 3a. 
Therefore, it becomes possible to make the size of display means such as the liquid crystal display portion 3a on the side of the display side casing portion 3 as large as possible and improve the performance thereof as much as possible. Consequently, it is possible to provide a radio communication device having display means capable of displaying document data and/or image data on a large display screen with high image quality.
Further, by employing the structure in which the antenna 10 is arranged on the side of the key side casing portion 4, it is possible to provide a radio communication device having a new design different from that of the conventional radio communication device 1 in which the antenna 10 protrudes from the upper end of the display side casing portion 3.
However, in the mobile communication device having the described antenna structure, which is arranged on the side of the key side casing portion, the helical portion of the antenna protrudes from in the vicinity of the hinge portion between the casing portions 3 and 4. Therefore, there is a problem that a user may touch the cap portion of the helical portion with his finger. In such case, gain of the antenna is degraded when the user performs a communication without extending the antenna.
This problem of the antenna structure in which the antenna is arranged on the side of the key side casing portion will be described in more detail with reference to FIG. 13 and FIG. 14.
FIG. 13 illustrates a case where the user holds the mobile communication device having the antenna structure shown in FIG. 12a to FIG. 12c provided on the side of the key side casing portion.
Further, FIG. 14 is a cross-sectioned front view of a main portion of the antenna structure of the mobile communication device shown in FIG. 12a to FIG. 12c. 
As shown in these figures, in the mobile communication device 1 having the antenna structure provided on the side of the key side casing portion 4 of the foldable casing 2 (FIG. 12a to FIG. 12c), the helical portion 12 and the cap 13 of the antenna 10 protrude from in the vicinity of the hinge portion 5 of the casing 2.
Therefore, when the user opens the casing 2 in order to use the mobile communication device 1, the user usually holds the key side casing portion 4. Consequently, his finger 100 may touch the surface of the cap portion 13 of the antenna 10 as shown in FIG. 13.
Therefore, when he uses the device without extending the antenna 10, transmission/receiving of electromagnetic wave through the helical portion 12 while his finger 100 touching the cap 13.
The helical portion 12 of the antenna 10 is covered by the cap 13 formed of synthetic resin, etc., and a wall thickness of the cap 13 is usually around 0.60 mm.
Therefore, when the finger 100 touches the surface of the helical portion 13, a distance d between the finger 100 and the helical portion 12 becomes 0.60 mm, which is equal to the wall thickness of the cap 13, as shown in FIG. 14.
If the distance d between the finger 100 and the helical portion 12 becomes about 0.60 mm, the antenna pattern and the antenna impedance of the antenna 10 are varied, and the electromagnetic wave is blocked by the finger 100, causing antenna gain to be degraded.
Therefore, in the mobile communication device 1 having the antenna 10 provided on the side of the key side casing portion 4 of the foldable casing 2, it is impossible to obtain good antenna gain. This problem is a fatal blow for the radio communication device.
On the other hand, the existence of the antenna of the conventional antenna structure in which the antenna is arranged on the side of the display side casing portion may constitute an obstacle to the display means for displaying image and/or document data, so that it becomes difficult to provide display means concomitant with the recent development of the data communication.
Therefore, a mobile communication device having an antenna arranged on the side of the key side casing portion and being free from degradation of antenna gain even when a user's finger, etc., touches the antenna structure and an antenna structure are strongly requested.